1. Field of the Invention
The present invention relates to a measuring apparatus and method for measuring the temperature, humidity, temperature distribution, or the like in a measurement region by radiating an optical signal onto the incidence end of an optical fiber arranged in the measurement region and analyzing backscattered light.
2. Description of the Related Art
Generally, when an optical signal having a frequency .omega. is radiated on a substance, the optical signal is scattered by the substance and a part of the scattered optical signal returns toward the radiation point. If this returning light is observed by optical time domain reflectometry (OTDR), light components having frequencies different from frequency .omega., e.g., frequencies .omega.+.omega.r, and .omega..+-.m.omega.r (m=an integer) are measured in addition to a light component having the frequency .omega., i.e., Raman scattered light is measured.
The intensity of Raman scattered light depends on temperatures. Since Raman scattered light is influenced by trace amounts of impurities or various molecules in an optical fiber, the application of Raman scattered light to temperature measurement was originally considered as difficult. However, with the progress of the manufacture and application techniques of optical fibers, use of optical fibers as thermometers has been studied.
A technique of using an optical fiber as a thermometer is disclosed, for example, in "New fiber optic distributed temperature sensor", SPIE Vol. 798 Fiber Optic Sensors II (1987) pp. 131 to 136, or in UK Patent Application GB 2140554A. The disclosures of these references are incorporated by reference in the specification.
In such a thermometer, backscattered light from each point of an optical fiber is weak, and noise is mixed in this weak optical signal. In addition, optical fibers slightly differ in characteristics from each other. For these reasons, it is very difficult to discriminate temperature measurement positions of an optical fiber and to accurately measure temperatures at the respective positions.
Under the circumstances, a technique of removing noise by averaging data obtained by radiating optical signals from thousands to tens of thousands of times is proposed. Even with this technique, however, high-fidelity measurement is very difficult to perform.
Even if measurement is performed by Optical Frequency Domain Reflectometory (OFDR) instead of OTDR accurate detection of a measurement position and accurate measurement of a temperature at the measurement position are difficult to perform for the same reasons as in measurement by OTDR.
In addition, in a conventional system, the intensities of backward Raman scattered light components are stored in an internal memory once, and data processing is subsequently performed. For this reason, unnecessary signals are stored in the internal memory. This undesirably increases the required memory capacity.